Communication systems based on modulated light sources are well known to the art. In high-speed communication systems, the light source is typically a laser. At frequencies below 10 GHz, the modulation can be imparted to the light source by turning the laser current on and off. Unfortunately, this type of laser modulation becomes increasingly difficult to implement as the modulation frequency is increased. To increase the laser modulation frequency the lasers have to be driven at high current densities which leads to reduced gain and increased damping from nonlinear effects. These nonlinear effects limit the modulation response of the laser.
Accordingly, light sources that are to be modulated at frequencies above 10 GHz are typically constructed by providing a laser that runs continuously and a separate light modulator that modulates the intensity of the laser output. The modulator typically has a transmissive state and an opaque state, which are switched back and forth by applying a potential across the modulator.
Modulators constructed from an electroabsorptive material are known to the art. These devices utilize a long waveguide having a p-i-n diode section whose transmission depends on the voltage placed across the device. While devices of this type can be modulated at frequencies in excess of 40 GHz, the voltage that must be applied across the device that is greater than 90 μm long to achieve this modulation frequency is in excess of 2-3 volts. Switching such large voltages at the frequencies in question presents problems that significantly increase the cost of such devices. In principle, a longer device can be utilized to provide the same on/off switching intensity at a lower voltage; however, there is a limit to the physical size of the device. The modulator has a capacitance that depends on its size. As the size is increased, the capacitance increases. At very high frequencies, the RC time constant associated with charging this capacitance limits the modulation response.